1. Field of the Invention
The present invention relates to a display device, and more particularly, to a display device with integrated in-cell type touch screen.
2. Discussion of the Related Art
Touch screens are a type of input device that is included in display devices such as Liquid Crystal Displays (LCDs), Field Emission Displays (FEDs), Plasma Display Panels (PDPs), Electroluminescent Displays (ELDs), and Electrophoretic Displays (EPDs), and enables a user to input information by directly touching a screen with a finger, a pen or the like while looking at the screen of the display device.
Particularly, the demand of display devices with integrated in-cell type touch screen, which include a plurality of built-in elements configuring the touch screen for slimming portable terminals such as smart phones and tablet Personal Computers (PCs), is recently increasing.
In a related art display device with integrated in-cell type touch screen disclosed in U.S. Pat. No. 7,859,521, a plurality of common electrodes for display are segmented into a plurality of touch driving areas and touch sensing areas, thereby allowing a mutual capacitance to be generated between the touch driving area and the touch sensing area. Therefore, the related art display device measures the change of a mutual capacitance that occurs in touch, and thus determines whether there is a touch.
In other words, in the related art display device with integrated in-cell type touch screen, a plurality of common electrodes for display perform the function of a touch electrode when a panel operates in a touch driving mode, for simultaneously performing a display function and a touch function.
In a related art in-cell type mutual capacitive touch screen using the existing common electrodes, a scheme that uses a plurality of driving electrodes and sensing electrodes necessary for touch driving temporally separates a display driving mode section and a touch driving mode section by using a common electrode, and thus prevents a noise ingredient (which occurs in the display driving mode section) from affecting the touch driving.
In the display driving mode section, a driving electrode and a sensing electrode act as common electrodes, and thus, a common voltage is applied to the driving electrode and the sensing electrode. Also, in the touch driving mode section, since the driving electrode and the sensing electrode act as touch electrodes, an alternating current (AC) voltage having a periodic driving pulse type is applied to the driving electrode, and a direct current (DC) voltage is applied to a touch IC in order for the touch IC to determine whether there is a touch by using a touch sensing signal which is generated between the driving electrode and the sensing electrode by a driving pulse.
FIG. 1 is a timing chart showing voltages which are respectively applied to a driving electrode and sensing electrode of a related art display device with integrated touch screen.
For example, as shown in FIG. 1, in a display driving mode section, a common voltage Z(V) is applied to both the driving electrode TX and the sensing electrode RX, and thus an equal voltage is generated between the driving electrode and the sensing electrode.
However, in a panel, the size and direction of the driving electrode differ from those of the sensing electrode, and thus, when the same common voltage is applied to the two electrodes in the display driving mode section, different influences are exerted on the driving electrode and the sensing electrode in a pixel charging/discharging operation performed by the turn-on/off of a switching transistor, causing a difference between voltages applied to liquid crystal.
As a result, in the display driving mode section, block dim is caused by a luminance difference between a driving electrode block and a sensing electrode block that are used as common electrodes.
Moreover, as shown in FIG. 1, in a touch driving mode section, a driving pulse that has X(V) as a high-level voltage VTX_HIGH and has a ground voltage as a low-level voltage VTX_LOW is applied to the driving electrode TX, and a touch sensing reference voltage VRX_REF of Y(V) that is a constant DC voltage is applied to a receiver of a touch IC connected to the sensing electrode RX.
Here, as shown in FIG. 1, the driving pulse is applied to the driving electrode in only some sections, and the ground voltage that is a low-level voltage is applied to the driving electrode in most sections.
Therefore, a difference between voltages respectively applied to the driving electrode and the sensing electrode occurs in the touch driving mode section, and thus, different voltages are applied to the driving electrode block and the sensing electrode block that are used as touch electrodes in the touch driving mode section, causing block dim due to a luminance difference between blocks similarly to the display driving mode section.